There is known a type of electric power steering system for a vehicle in which the rotational output of an electric motor is reduced in speed through a worm gear mechanism to assist in driving an output shaft connected to a steering wheel. In a known type of power steering system for light cars to which no heavy load is applied, a combination of a metallic cylindrical worm and a worm wheel made of a resin is used as a worm gear mechanism [for example, Japanese Patent Application Unexamined Publication (KOKAI) No. Hei 9-24855].
Because it requires accuracy, the metallic worm is produced by turning quench-hardened steel in a lathe, followed by heat treatment and finish grinding. When turned in a lathe, the workpiece is cut with a cutting tool. However, when it is desired to increase productivity, a conical milling cutter is used, and the workpiece is cut in a thread cutting manner with the axis of the milling cutter inclined with respect to the worm axis by γ (the lead angle at the pitch line of the worm).
However, the above-described worm manufacturing process needs roughly at least three steps, i.e. cutting, heat treatment, and grinding. Regarding equipment therefor, also, at least three apparatuses are required, i.e. a lathe, heat-treatment equipment, and a grinding machine. For this reason, the processing cost increases, and the advantage of the resin worm wheel cannot satisfactorily be exhibited.
Meanwhile, when an external thread, a worm or the like is formed by rolling, a first rolling die and a second rolling die, which are disposed facing each other, are fed toward each other to penetrate the work material. At this time, when the lead angle of the worm is large and there is a large difference between the finished diameter and the blank diameter, the workpiece may undesirably travel during the progress of rolling process, causing a change in the lead angle. Such a phenomenon is known as “through-feed”. If through-feed occurs, the condition of contact of the cylindrical dies becomes different between the flank of thread in the direction of travel of the workpiece due to the through-feed and the opposite flank, resulting in degradation of the finished accuracy of the rolled surface. To prevent the occurrence of through-feed, the conventional practice is to make correction by changing the phase position in the axial direction of the first or second rolling die on the basis of visual observation or the like.
However, this correction method for preventing through-feed is difficult to carry out for a component part in which there is a shaft on each side of an external thread or a worm, the shaft having a larger diameter than that of the external thread or the worm, because the rolling die may interfere with the shaft. In such a case, the dies may be rotated in the forward or reverse direction to perform rolling. With this method, however, high product accuracy cannot be obtained owing to the occurrence of backlash or the like, and productivity is inferior. The present applicant proposed a spindle tilting mechanism that causes the first and second rolling dies to pivot about an axis perpendicular to the axes of rotation of the dies to prevent the occurrence of through-feed [Japanese Patent Application Unexamined Publication (KOKAI) No. Hei 11-285766].
However, even if the spindle tilting mechanism is used, the occurrence of through-feed cannot completely be prevented in the case of a workpiece for a worm or the like that has a large change in diameter during the rolling process from the initiation to the termination of the process, resulting in occurrence of a working error.
An object of the present invention is to provide a method of forming a worm by rolling that allows a reduction in the number of processing steps while ensuring the worm accuracy satisfactorily, and also provide a worm formed by the rolling method.
Another object of the present invention is to provide a method of forming a worm by rolling that allows a reduction in costs while ensuring the worm accuracy satisfactorily, and also provide a worm formed by the rolling method.
The advantages of the present invention are as follows. The present invention allows an axially shifted, thin-toothed worm to be formed with high accuracy by rolling. Accordingly, the number of processing steps required can be reduced in comparison to the conventional worms. Moreover, the processing cost can be reduced to a considerable extent.